1. Field of the Invention
The invention relates to a locking device for securing a rod-shaped element in a receiving part of a bone anchor and to a bone anchor with such a locking device. The locking device includes a first locking element and a second locking element that is positionable in the first locking element and that is protected against backing out or unintended removal from the first locking element. The invention is particularly applicable to a polyaxial bone screw, where a head of a screw element and the rod-shaped element can be fixed independently from one another.
2. Description of Related Art
A polyaxial bone screw wherein the head of the screw element and the rod can be fixed independently is known, for example, from U.S. Pat. No. 7,223,268 B2. The locking device described therein has a first locking element cooperating with a thread provided at the inner wall of the receiving part and a second locking element in the form of a set screw that is contained in the first locking element.
A nested fastener and set screw combination for securing a spinal fixation rod to a bone screw, is described in U.S. Pat. No. 7,204,838 B2. The fastener base has a central threaded bore to receive a threaded set screw. The fastener base is provided with a radially inwardly extending abutment shoulder to engage and abut the set screw, prohibiting advancement of the set screw out of a top of the fastener and allowing for a simultaneous removal of the set screw and the fastener.